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The establishment of functional retinal circuits in the mammalian retina depends critically on the proper generation and assembly of six classes of neurons, five of which consist of two or more subtypes that differ in morphologies, physiological properties, and/or sublaminar positions. How these diverse neuronal types and subtypes arise during retinogenesis still remains largely to be defined at the molecular level. Here we show that all four family members of the early B-cell factor (Ebf) helix-loop-helix transcription factors are similarly expressed during mouse retinogenesis in several neuronal types and subtypes including ganglion, amacrine, bipolar, and horizontal cells, and that their expression in ganglion cells depends on the ganglion cell specification factor Brn3b. Misexpressed Ebfs bias retinal precursors toward the fates of non-AII glycinergic amacrine, type 2 OFF-cone bipolar and horizontal cells, whereas a dominant-negative Ebf suppresses the differentiation of these cells as well as ganglion cells. Reducing Ebf1 expression by RNA interference (RNAi) leads to an inhibitory effect similar to that of the dominant-negative Ebf, effectively neutralizes the promotive effect of wild-type Ebf1, but has no impact on the promotive effect of an RNAi-resistant Ebf1. These data indicate that Ebfs are both necessary and sufficient for specifying non-AII glycinergic amacrine, type 2 OFF-cone bipolar and horizontal cells, whereas they are only necessary but not sufficient for specifying ganglion cells; and further suggest that Ebfs may coordinate and cooperate with other retinogenic factors to ensure proper specification and differentiation of diverse retinal cell types and subtypes. [ABSTRACT FROM AUTHOR]

In the mammalian retina, amacrine cells (ACs) contain numerous subtypes with extremely diverse morphologies and physiological functions. To date, how these subtypes arise during retinogenesis remains largely unknown at the molecular level. The orphan nuclear receptor Nr4a2 plays an essential role in specifying ventral midbrain dopaminergic neurons, and its mutations are associated with familial Parkinson's disease. Here we show that Nr4a2 is also critically involved in the specification of AC subtype identity. During mouse retinogenesis, Nr4a2 is expressed in a subset of postmitotic GABAergic ACs and their precursors. Its targeted inactivation results in the loss of a subpopulation of GABAergic ACs that include all dopaminergic and p57Kip2+ neurons as well as a simultaneous increase of calbindin+ ACs. Misexpressed Nr4a2 can promote GABAergic AC differentiation and repress calbindin+ ACs, whereas its dominant-negative form has the ability to suppress the GABAergic AC fate. Moreover, the expression of Nr4a2 is positively regulated by Foxn4 and negatively controlled by Brn3b, two retinogenic factors previously shown to promote and suppress GABAergic ACs, respectively. These data suggest that Nr4a2 is both necessary and sufficient to confer AC precursors with the identity of a GABAergic AC phenotype, and that it may network with multiple other retinogenic factors to ensure proper specification and differentiation of AC neurotransmitter subtypes. [ABSTRACT FROM AUTHOR]

The retinal ganglion cells (RGCs) are the sole output neurons in the retina that form the optic nerve and convey light signals detected by photoreceptors to the higher visual system. Their degeneration and damage caused by glaucoma and injury can lead to blindness. During retinogenesis, RGCs are specified from a population of multipotential precursors capable of generating RGC, amacrine, horizontal, and cone cells. How the RGC fate is selected from these multiple neuron fates is unknown at present. Here we show that the previously unsuspected POU domain transcription factor Brn3b (brain-specific homeobox/POU domain protein 3b) plays such a critical role. Loss of Brn3b function in mice leads to misspecification of early RGC precursors as late-born RGC, amacrine, and horizontal cells, whereas misexpressed Brn3b suppresses non-RGC cell fates but promotes the RGC fate. Microarray profiling and other molecular analyses reveal that, in RGC precursors, Brn3b normally represses the expression of a network of retinogenic factor genes involved in fate commitment and differentiation of late-born RGC, amacrine, horizontal, and cone cells. Our data suggest that Brn3b specifies the RGC fate from multipotential precursors not only by promoting RGC differentiation but also by suppressing non-RGC differentiation programs as safeguard mechanism. [ABSTRACT FROM AUTHOR]